Sunday 30 September 2012 17.30 EDT
First published on Sunday 30 September 2012 17.30 EDT

Two female glow-worms have been sent to me, with the request – can I explain how they differ from the larval insect? Of course the larva or "grub" stage is wingless, but the female is also devoid of these useful limbs or appendages: indeed she very closely resembles the larva. The actual head of the glow-worm is small, hidden below the prothorax, except when the insect is feeding, but what appears to be the head, this prothorax, is much larger and shield-shaped in the female than the first segment of the younger insect. If we look at a larva and an imago together we shall see that the legs in the latter are stronger and longer, and also that the adult insect shows a much brighter light than the other. It is said that the female, lacking the ability to fly to the winged male, shows her candle to lure him. This may explain the brighter light, but it is not the only explanation of the light, for not only has the male a luminous spot, but a small shine may be shown by the larva and even the sleepy pups. Indeed, I found that a pupal glow-worm seems to have no control over its light, and by a slight irritation I could at any time, by night or day, cause it by reflex action to switch on its tiny lamp.

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